RFR: 8215118: Reduce HeapRegion to only have one member for index in CSet [v3]

[Ivan Walulya]

> Hi all,
> 
> Please review this change to maintain only a single member variable to indicate the region's index in a collection set: either the actual collection set for young regions or the optional regions array if the region is considered optional during a mixed collection.
> 
> Testing: Tier 1-7

Ivan Walulya has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - resolve conflict
 - after kstefanj discussion
 - 8215118: Reduce HeapRegion to only have one member for index in CSet

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Changes: https://git.openjdk.java.net/jdk/pull/5122/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5122&range=02
  Stats: 54 lines in 9 files changed: 11 ins; 10 del; 33 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5122.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5122/head:pull/5122

PR: https://git.openjdk.java.net/jdk/pull/5122